This invention relates to voltage-controlled oscillators, and in particular to a current-regulated voltage-controlled oscillator, for example, for application to phase-locked loop circuits.
Numerous phase-locked loop circuits and voltage-controlled oscillator circuits are well known in the art. Such circuits are widely used as clock generators in microprocessors or other digital devices. One well known approach is described in "A 320 MHz, 1.5 mW at 1.35 V CMOS PLL for Microprocessor Clock Generation," by von Kaenel, V., et al., 1996 International Solid-State Circuits Conference, "Digital Clocks and Latches," Session 8, Paper FA 8.2, pp. 132-133. The phase-locked loop circuit described in this paper is intended to operate continuously at low power. Because it is on the same chip as the microprocessor, the power supply switching noise for the digital circuits is controlled to minimize jitter. The voltage-controlled oscillator shown in that paper is a typical circuit for such devices. Unfortunately, phase-locked loop devices using voltage-controlled oscillators, such as depicted, use a resistor to implement loop stabilization and to control the damped nature of the transient response. Secondly, the voltage-controlled oscillator is usually operated at twice the ultimately desired clock speed. This effects the power supply noise rejection. Furthermore, such devices suffer from a dependence upon the process technology used to fabricate the device. The process technology will change the resistance of a resistor (see LF in FIG. 1 of the above paper) in the circuit. As the resistance varies, the stability of the loop and the transient response damping may both be compromised.
For these reasons, as well as others, an improved circuit is desired.